Mrs B has an ultegra compact chainset that is 50/34. Can we fit sub compact chainrings? is it possible to go 46/30 or will i need something like Miche Graff subcompact chainset to get those ratios?
thanks

34 is the smallest that you can go on that chainset.

Shimano have just released a range of chainsets (GRX) with smaller chainrings – there are 48/31 and 46/30 options. No availability yet, looking like next month at the earliest. They will also require a new front mech to work properly.

Absolute black have 46/30 rings.
The listing says compatable with 6800.
https://absoluteblack.cc/oval-road-chainrings-30-46-and-32-48-for-110-4bcd/

If its 110 BCD then 34 is the smallest standard ring, Praxis make a 32t but I’m not sure if it’s for their own crank or available for shimano bolt pattern

I’ve been using the Absolute Black 30/46 combo on a 105 4 arm chainset which is the same setup as the Ultegra 8000 and it works fine. Survived the Turin Nice Rally no problem and I was glad of the lower gearing fully laden on the Col d’Assietta. Works with a standard front mech and has the added benefit of being oval so smoothing the pedal stroke and knee issues.

Ah, they offset the chainrings to make them work. Clever.

As above Absolute Black do them. I’d wanted to do the same on my road bike and go to a 30/46 and change the cassette to a 11-28 from a 11-32. Unfortunately my Shimano chainset is 5 arm. I’ll probably wait till the rings are worn on that then swap it out for the GRX and a new front mech. Either that or a Praxis one with a new BB

The AB ones seem pretty expensive. The Miche Graff is cheaper as an entire chainset with rings. They do look nice though.
Changing the FD would be a pain as her bike is Di2.

Beener

I’d not seen the Miche option when I was looking, seems to be the easiest option for me at it looks like it’ll fit my BB86 bottom bracket and won’t need a new front derailleur. As a bonus they also do it with 165 cranks which I prefer

Ah, they offset the chainrings to make them work. Clever.

Well almost very clever. I got some for my bike (Shimano 105) and the 2.5mm offset meant the inner ring was too far inboard and I did not have enough adjustment in my front deraillieur to accommodate. I contacted AB who were very helpful, but unfortunately the only advice they could offer was to space out the crank or BB, which I couldn’t do as it was a press fit BB. So double check your compatibility. I think the issue with my bike is the BB area of the frame is very thick with ovalised tubing as it all goes into the BB area junction, which probably positions the front deraillieur a bit further outboard than normal.

But no worries, a cheaper and more effective option for me was to buy the latest Shimano 105 rear mech that has a 34T capacity and went for a 34T cassette so I achieved the 1:1 first gear ratio that way (34:34 instead of 32:32). In reality I think my previous 105 mech with 32T max capacity would have probably worked fine with the 34T cassette, but didn’t want to push it and 105 rear mech’s are cheap enough.